I have found the silver bullet, the proof that the idea for the Secret Forest robbers did occur to EB while writing the Secret Mountain (and I did not even have to ask Tony to look into his EB archive!):
The secret Mountain Chapter ten wrote: “Walk through the rock!” said Jack. “That sounds a bit like Ali Baba and the Forty Thieves. Do you remember -- the robbers made their home in a cave inside a hill -- and when the robber chief said ‘Open Sesame!’ a rock slid aside -- and they all went in!”
The secret Mountain Chapter twelve wrote: “It not only swung round, it slid to one side,” said Ranni. “Just like the rock in the story of Ali Baba that you told me! And behind it was a great door in the mountain-side studded with shining knobs that glittered in the sun!”
This is the very idea that forms the central plot of The Secret of Killimooin:
The secret of Killimooin Chapter ten wrote: Jack was so amazed that he could not say a word. The statue split completely in half, the two halves moving right apart -- and then, from the floor of the flat rock beneath, a man’s shaggy head appeared, full in the moonlight -- the head of one of the robbers!
It did not occur exactly as I had thought - not a full manuscript or fears of racial insensitivity - but when EB developed this idea in The Secret of Killimooin, she did include elements closely connected to the previous story. I have already noted the names Tooku and Yamen appearing to be African (the setting of the previous story), and the clothing and lack of clothing of the goatherd Beowald.
The two stories are similar in structure, more so than usual. This is especially true with the characters of Mafumu and Beowald. Beowald and his goats take the place of Mafumu and his uncle. Beowald is Mafumu. The little goats - the kids - take the place of Mafumu’s playfulness, and the older goat with the curled horns is Mafumu’s uncle (Beowald refers to it as “old one”). It is this goat which actually leads the way further up the mountain to overlook the Secret Forest. This is Mafumu’s uncle, the guide who took the party to the spot between the two other mountains to see the Secret Mountain in the distance. And even when at the end they go back to the castle in the mist, it is this goat who leads them and Beowald. But Beowald is much older than Mafumu - he is a youth on the verge of manhood. And he is in charge of his “uncle” the goat! Whose “spears”, now curled horns, are blunted. We had had a premonition of this at the very end of Secret Mountain, when Ranni had thought that Mafumu was brave and smart enough to be chief of his village one day, and when Mafumu had strutted off with all his gifts and the villagers, including the uncle, had all run scared from him.
When Beowald goes down into the tunnel to search for the children, even though he is blind and is especially fearful since he is in unfamiliar terrain, that is Mafumu when he finds a way into the Secret Mountain to rescue the others by swimming under water. Both are enveloped in darkness.
Beowald lived in a double darkness - he slept during the day and was awake at night. But we do have a clue to the origin of Beowald’s blindness. At the end of the Secret Mountain, one of the gifts that Mafumu gets is a pair of sunglasses. Beowald’s blindness also gives him the dark complexion of Mafumu.
Also, there is a desire to continue the relationship with both Beowald and Mafumu. Paul wants to reward Beowald by having him come to live and work at the palace, and the children express the wish to bring Mafumu back to England. The same conclusion is reached in the end, that both those things would make Beowald and Mafumu unhappy to leave their homes.
Also a remarkable amount of attention is paid to the feet of Mafumu and Beowald. Mafumu hurts his bare foot when he gets a thorn in it, which is removed and the foot bandaged by Ranni and Pilescu. And Beowald’s bare feet are mentioned quite a bit. He himself refers to the pebbles under his feet (Mafumu also caught his foot on a stone which leads to an uproar), plus the attention that the footprints are given in the cave when it is thought that they might be those of Beowald’s bare feet.
Also Beowald is found by means of a crooked tree. And the payment that Mafumu’s uncle receives is hidden under a tree by Ranni, the same place where the children had first met Mafumu.
Jack is also quite rude to Beowald at one point, making an off-putting remark, recalling the incident where he was angry at Mafumu’s uncle for hitting Mafumu.
Beowald is unusually fearful of the robbers, recalling Mafunu and his uncle’s fear of the Mountain Folk.
We even have a repeat of the dyeing of body parts. The Secret Forest robbers dyed their wolf tail red as the Mountain Folk did their hair.
The flute that Beowald plays his beautiful music on, and which evokes the surrounding mountainside, is the fruits and flowers that Mafumu collects for the children from the forest, and which they eat.
Both Beowald and Mafumu give gifts associated with the lips (“kisses” or “food”) - Beowald gives a flute to Paul, and Mafumu gives his charm of crocodile teeth, both flute and charm worn round the neck.
The circumstantial evidence for the name Killimooin being derived from that of Kilimanjaro is even stronger because the similar evidence for Kilimanjaro in Tanzania being the inspiration for the location of the Secret Mountain is quite strong, and we have shown that the idea for the Secret of Killimooin did come from the Secret Mountain, referring directly to (the substitute for) kilimanjaro.
Of course, the original idea for the robbers came from the Middle East, being the story of
Ali Baba and the Forty Thieves, a tale in The Arabian Nights (a surprisingly mature tale for children!). But the society of Ali Baba and the Forty Thieves was definitely not Stone Age. Those elements would have been inspired by somewhere else, and most likely Africa.
None of this excludes the possibility that the European setting for Killimooin could be the Carpathian Mountains in Romania.